The present invention relates to an organic lubricating coating composition suitable for use in plastic deformation of a metal sheet. More particularly, the present invention relates to an organic lubricating coating composition capable of significantly improving the formability of a metal sheet during plastic deformation and having improved removability from the metal sheet by washing with an aqueous washing solution after plastic deformation. The present invention also relates to a lubricated metal sheet coated with such a coating composition.
In the forming of a metal sheet such as a steel sheet by plastic deformation using a die, a lubricant is conventionally applied to the metal sheet in order to increase slippage between the metal sheet and the die, thereby reducing the required forming load and preventing galling of the metal sheet. Recently, however, in view of environmental and other problems involved in the application of a high viscosity lubricant to a metal sheet, lubricated metal sheets having a solid lubricating coating already formed at the sheet-making stage or a subsequent appropriate stage have been widely used.
The lubricating coating on lubricated metal sheets is usually formed form an organic lubricating coating composition comprising one or more lubricants and one or more organic film-forming resins.
Organic film-forming resins which have been used to form a lubricating coating include acrylic resins, epoxy resins, melamine resins, phenolic resins, and the like. As the lubricants, fatty acids, fatty acid alkali metal salts, metallic soaps, fatty acid esters, fatty acid amides, fatty alcohols, polyethylene powder, graphite, molybdenum disulfide, fluoroplastic powder, etc. have been used.
For example, Japanese Patent Application Kokai No. 62-84193(1987) discloses an aqueous dispersion of an acrylic resin and a metallic soap as a lubricating coating composition. A combination of wax with a water-soluble resin is described in Japanese Patent Publication No. 52-22948(1977). A combination of a polymethacrylate resin and a mineral oil is disclosed in Japanese Patent Application Kokai No. 54-40259(1979), while a combination of a thermosetting acrylic resin, surfactant and wax is described in Japanese Patent Application Kokai No. 63-8489(1988).
An organic lubricating coating composition for plastic deformation of a metal sheet, particularly for use in the preparation of lubricated metal sheets must be able to form a coating having the following properties:
(b 1) good lubricating properties even with a thin coating;
(2) non-tackiness or anti-blocking when a coated lubricated metal sheet contacts another coated lubricated metal sheet; and
(3) easy removal by washing with an aqueous washing solution since it must be removed after plastic deformation.
Most of the conventional organic lubricating coating composition for use in plastic deformation do not meet all the above-mentioned requirements sufficiently.
Lubricating coating compositions based on wax, which are known to have good lubricating properties, must be applied after they are diluted with an organic solvent. However, the use of an organic solvent makes handling troublesome and is undesirable from an environmental viewpoint. Furthermore, these coating compositions suffers another problem that removal of a coating formed therefrom is difficult.
In order to eliminate these problems, it has bee proposed to use an aqueous dispersion or emulsion of wax. Such a dispersion or emulsion can be applied to a metal sheet in the absence of an organic solvent, but due to the strong hydrophobic nature of the wax, it is difficult to remove the resulting lubricating coating by washing with an aqueous washing solution. Therefore, after forming the metal sheet, the lubricating coating must be removed by washing under severe conditions such as by washing with an organic solvent or a boiling alkaline solution.
Lubricating coating compositions based on a water-soluble resin or a water-dispersible resin such as an acrylate resin have the advantage that a coating thereof can be easily removed due to the hydrophilic nature of the resin. However, such a resin does not have a long-chain alkyl or alkenyl group in the molecule which contributes to lubricity of the resin. Therefore, the thickness of the coating must be increased in order to achieve an adequate degree of lubricity. In some cases it is necessary for the lubricating coating to have a thickness as large as several tens of micrometers, which is highly disadvantageous from an economical viewpoint.